The United States Marine Corps (USMC) is a branch of the United States armed forces responsible for providing force projection from the sea, using the mobility of the United States Navy to rapidly deliver combined-arms task forces. It is one of seven uniformed services of the United States. In the civilian leadership structure of the United States military, the Marine Corps is a component of the United States Department of the Navy, often working closely with U.S. naval forces for training, transportation, and logistic purposes; however, in the military leadership structure the Marine Corps is a separate branch.
Captain Samuel Nicholas formed two battalions of Continental Marines on 10 November 1775 in Philadelphia as naval infantry. Since then, the mission of the Marine Corps has evolved with changing military doctrine and American foreign policy. The Marine Corps has served in every American armed conflict and attained prominence in the 20th century when its theories and practices of amphibious warfare proved prescient and ultimately formed the cornerstone of the Pacific campaign of World War II. By the mid-20th century, the Marine Corps had become the dominant theorist and practitioner of amphibious warfare. Its ability to respond rapidly to regional crises gives it a strong role in the implementation and execution of American foreign policy.
The United States Marine Corps includes just over 203,000 active duty Marines (as of October 2009) and just under 40,000 reserve Marines. It is the smallest of the United States' armed forces in the Department of Defense (the United States Coast Guard is smaller, about one-fifth the size of the Marine Corps, but is normally under the Department of Homeland Security). The Marine Corps is nonetheless larger than the armed forces of many significant military powers; for example, it is larger than the active duty Israel Defense Forces, or the entire British Army.